Conventionally, a vehicle provided with a side door, also called a double-leafed hinged door type side door, at the side portion of the car body has been put into practical use. In the double-leafed hinged door type vehicle, side openings with center pillars are respectively formed on two side surfaces of the car body. A front hinge is arranged at the front end of each side opening, so one can open and close the front door about this hinge. A rear hinge is arranged at the rear portion of each side opening, so one can open and close the front door about this hinge. The front and rear doors are locked by strikers provided to the center pillar, so they remain closed.
For example, according to Japanese Patent Laid-Open No. 10-109662, double-leafed hinged door type front and rear doors are assembled at a side portion of a car body. In this vehicle, in order that one can open and close the front door with the rear door being closed, the whole car body is composed of a skeleton member. A side surface opening is formed of side skeleton members. A front door whose front end is installed to a front skeleton member to form the side surface opening through hinges is provided. A rear door whose its rear end is installed to a rear skeleton member to form the side surface opening through hinges is also provided.
In recent years, a technique is proposed with which automobile components having complicated shapes can be obtained by hydroforming. According to this technique, a pipe-shaped material is prepared, is set in a mold, and is shaped or formed along the contour of the female die of the mold by increasing the inner pressure of the material. According to the technique disclosed in Japanese Patent Laid-Open No. 9-150752, an aluminum alloy pipe is bent three-dimensionally to form hoops. The hoops are subjected to warm blow forming to obtain frame bodies. These frame bodies are combined to complete a car body frame. Therefore, little scraps are generated from a given amount of material, so the yield improves.